For easy driving of a vehicle and for reducing fatigue to the driver, various kinds of power transmission devices for vehicles have, in recent years, been widely used for easy driving. A representative example will be a so-called automatic transmission (AT) combining a torque converter and a planetary gear device together. A power transmission device which uses a transmission of the type of parallel shaft gear mechanism similar to the so-called manual transmission, in combination with an automatic clutch, is one of such automatic power transmission devices for vehicles. In this power transmission device, a clutch disposed between an engine and a transmission is provided with a clutch actuator, and the clutch is automatically connected or disconnected at the time when the driver changes the speed by shifting the gear by using a shift lever or at the start of the vehicle eliminating the need of operating the clutch pedal by the driver. There has been further proposed a power transmission device which automatically shifts the gear depending upon the traveling condition of the vehicle by using an electronically controlled device obviating the need of operating the shift lever by the driver.
A clutch (dry type single disk clutch) installed between an engine and a transmission is provided with a clutch disk 101 which has a friction disk fixed to the peripheral portion thereof as shown in FIG. 6. The clutch is slidably fitted by spline to a transmission input shaft 103 rotatably supported by a crankshaft 102 of the engine. A pressure plate 105 is provided on the back of the friction disk of the clutch disk 101 to bring the friction disk into pressed contact with a flywheel 104 at the rear end of the crankshaft 102. Further, a diaphragm spring 107 is attached to a clutch cover 106 that is fixed to the flywheel 104. When the vehicle is normally traveling, the diaphragm spring 107 brings the clutch disk 101 into pressed contact with the flywheel 104 via the pressure plate 105 and, therefore, the engine power is transmitted to the transmission input shaft 103 via the clutch disk 101.
The clutch is equipped with an operation mechanism for connecting and disconnecting the transmission of power, and the operation mechanism is constituted by a release bearing 108, a release fork 109, a clutch actuator 110 and the like. The clutch actuator 110 is a fluid pressure cylinder operated by a pneumatic pressure or a hydraulic pressure, and its piston is coupled to one end of the release fork 109. Provision is, further, made of a stopper 111 for mechanically limiting the movement in order to prevent the occurrence of damage to the clutch actuator 110 and the like caused by excessively large movement of the piston.
At the time of cutting off the engine power for shifting the gear of the transmission, the working fluid is fed to the clutch actuator 110 to displace one end of the release fork 109 toward the right in the drawing. The other end of the release fork 109 displaces toward the opposite direction, causing the release bearing 108 coming in contact therewith to slide leftward so that the diaphragm spring 107 moves as represented by a two-dot chain line in the drawing. Therefore, the spring force that pushes the pressure plate 105 is released, and the transmission of the engine power to the transmission input shaft 103 is cut off. To connect the clutch again after having finished the gear shift, the working fluid in the clutch actuator 110 is discharged, and the release fork 109 is moved leftward by a return spring 112 or the like. The state of connecting the clutch (rate of connection) is determined by the movement of the piston of the clutch actuator 110, i.e., by the stroke of the clutch actuator.
At the time of gear shifting, the clutch must be disconnected and connected quickly without causing shift shock. Therefore, at the time of connecting again the clutch that is once disconnected after having shifted the gear (after the gears are engaged), the piston of the clutch actuator 110 is, first, quickly moved in a direction of connection so as to quickly pass through an invalid region where the torque is not substantially transmitted, and the rate of connection is gradually increased in the so-called half-engage clutch region where the torque starts transmitting in order to avoid the shift shock caused by a sharp increase in the rate of connection as illustrated in a graph of FIG. 7 that shows changes in the stroke. The above control is executed by varying the amount of the working fluid in the clutch actuator 110 to correctly control the stroke thereof.
A clutch control device which automatically connects and disconnects the clutch at the time of gear shifting is provided with a working fluid pressure source such as an air tank that feeds the working fluid, a stroke sensor for detecting the movement of the piston of the clutch actuator, and control valves for controlling the amount of the working fluid in the clutch actuator. The clutch control device executes the clutch control at the time of gear shifting. Usually, the control valves are arranged in the working fluid feed pipe and in the discharge pipe, respectively. The rate of connection of the clutch is controlled by opening and closing these two control valves. There has also been known a clutch control device which feeds and discharges the working fluid in the clutch actuator by using a single flow rate control valve as disclosed in, for example, Japanese Patent No. 3417823.
In the clutch control device that uses a single flow rate control valve as shown in a circuit diagram of FIG. 8, the flow rate control valve 1 is connected to a communication passage 2 communicated with the clutch actuator 110, to a pressure source passage 4 communicated with the working fluid pressure source 3 such as an air tank, and to a discharge passage 5 for discharging the working fluid from the clutch actuator 110 and, further, includes three ports, i.e., a communication port 2p, a pressure source port 4p and a discharge port 5p formed therein and opened to the respective passages.
The flow rate control valve 1 of FIG. 8 is a proportional control valve of the type of slide valve equipped with a drive device of the type of electromagnetic solenoid, and works as a valve actuator for operating a valve body 6. Namely, the flow control valve 1 has such flow rate characteristics that the flow rate of the working fluid that flows therethrough varies depending upon the position of the valve body 6. The amount of electric current flowing into the electromagnetic solenoid serves as an operation amount for varying the flow rate. To control the stroke of the clutch, a flow rate control valve control device 9 is connected to the flow rate control valve 1 to control the amount of electric current to a coil 8 responsive to a detection signal from a stroke sensor 7.
As shown in detail in the operation view of FIG. 9, the valve body 6 of the flow rate control valve 1 has two lands on the way thereof, one end of the valve body 6 being coupled to a moving yoke 10 of the electromagnetic solenoid. A spring 11 is arranged at the other end of the valve body 6, and the position of the valve body 1 is determined by a balance between the magnetic force acting on the moving yoke 10 and the resilient force of the spring 11. When the flow of current to the coil 8 is interrupted (amount of current, 0%), the valve body 6 is pushed by the spring 11 and assumes a position shown in FIG. 9(b) whereby the communication port 2p communicates with the discharge port 5p, and the working fluid in the clutch actuator 110 is discharged to the exterior permitting the clutch to be connected. If the electric current flowing into the coil 8 assumes a maximum value (100%), the valve body 6 is brought to a position shown in FIG. 9(c) compressing the spring 11, and the communication port 2p communicates with the pressure source port 4p. Therefore, the working fluid in the pressure source 3 is introduced into the clutch actuator 110 through the communication port 2p, and the clutch is disconnected. When a 50%-current flows into the coil 8, the valve body 6 is brought to a position of FIG. 9(a), i.e., brought to the neutral position, and the communication port 2p is cut off from the power source port 4p and the discharge port 5p; i.e., the stroke of the clutch is maintained at a predetermined position. To control the stroke of the clutch at the time of gear shifting as shown in FIG. 7, the amount of electric current flowing into the coil is so controlled as to vary according to a pattern shown on the lower side in FIG. 7.
Here, described below is a relationship between the position of the valve body of the flow rate control valve and the flow rate. In the flow rate control valve in which the length L of the land is the same as the width W of the communication port 2p, the working fluid readily starts flowing if the valve body is deviated toward the right or the left from the neutral position in FIG. 9(a). The flow rate control valve has only one neutral position at which the flow rate becomes 0, and has flow rate characteristics as represented by a two-dot chain line in FIG. 11, i.e., flow rate characteristics that are symmetrical on the feed side and on the discharge side with the neutral position (amount of current, 50%) as a center. When the above flow rate control valve is used, the amount of the working fluid in the clutch actuator 11 readily varies if deviated from the neutral position. To maintain the stroke at a predetermined position, therefore, it becomes necessary to accurately control the electric current that flows into the coil 8.
On the other hand, if the length L of the land is set to be larger than the width W of the communication port 2p by only a small amount as shown in FIG. 10(a), then a small width is imparted to the neutral position of the flow rate control valve 1. In this case, the flow rate characteristics become as represented by a solid line in FIG. 11, the neutral position includes an dead zone DZ where the flow rate does not change despite the operation amount is varied, and the flow rate becomes symmetrical on the feed side and on the discharge side relative to the central point. Therefore, despite the electric current varies to some extent due to disturbance or the like while the electric current flowing into the coil 8 has been so set as to maintain the stroke at the predetermined position, the stroke does not vary and a stable operation is realized. The dead zone DZ of the flow rate control valve 1 may also occur due to unavoidable error during the production in the step of producing the flow rate control valves.
In the flow rate control valve which includes the dead zone DZ, the position where the valve body 6 is brought to a halt so as to maintain a predetermined stroke while the stroke of the clutch is being controlled, varies in the direction in which the valve body 6 moves. When the valve body 6 moves toward the right in FIG. 10 to arrive at the neutral position, i.e., when the working fluid is fed to the clutch actuator 110 to operate the clutch in the direction of disconnection (stroke increases) and, thereafter, the valve body 6 is brought to the neutral position to cut off the feed, the valve body 6 stops at a position of FIG. 10(b)(corresponds to a point P in FIG. 11) where the right end of the land of the valve body 6 closes the communication port 2p. Conversely, when the valve body 6 moves toward the right in FIG. 10 to arrive at the neutral position, i.e., when the working fluid is discharged from the clutch actuator 110 to operate the clutch in the direction of connection (stroke decreases) and, thereafter, the valve body 6 is brought to the neutral position to stop the discharge, the valve body 6 stops at a position of FIG. 10(b)(corresponds to a point N in FIG. 11) where the left end of the land of the valve body 6 closes the communication port 2p. 